U.S. patent application number 12/056671 was filed with the patent office on 2008-10-16 for image forming apparatus.
Invention is credited to Toshiaki Ino, Yasuhiro NISHIMURA, Akiko Tsuji.
Application Number | 20080253780 12/056671 |
Document ID | / |
Family ID | 39853821 |
Filed Date | 2008-10-16 |
United States Patent
Application |
20080253780 |
Kind Code |
A1 |
NISHIMURA; Yasuhiro ; et
al. |
October 16, 2008 |
IMAGE FORMING APPARATUS
Abstract
An image forming apparatus including: an image forming section
for forming an image, the image forming section including a
photoconductor, a charging unit, a developing unit, a toner supply
unit which supplies a toner to said developing unit, and a
developing bias power supply section which supplies a developing
bias voltage to said developing unit; and a controller that
activates said image forming section, determines a target value of
a charging potential of said photoconductor and/or a developing
bias voltage, for forming the image, and controls the charging unit
and/or the developing bias power supply section in accordance with
a determined result, wherein when an absolute value of the
determined target value of the charging potential or an absolute
value of the determined developing bias voltage is larger than a
prescribed value, said controller controls replacement of a
prescribed amount of a toner in the developing unit.
Inventors: |
NISHIMURA; Yasuhiro; (Osaka,
JP) ; Ino; Toshiaki; (Souraku-gun, JP) ;
Tsuji; Akiko; (Kameyama-shi, JP) |
Correspondence
Address: |
NIXON & VANDERHYE, PC
901 NORTH GLEBE ROAD, 11TH FLOOR
ARLINGTON
VA
22203
US
|
Family ID: |
39853821 |
Appl. No.: |
12/056671 |
Filed: |
March 27, 2008 |
Current U.S.
Class: |
399/27 ; 399/50;
399/55 |
Current CPC
Class: |
G03G 15/0266 20130101;
G03G 15/0822 20130101; G03G 15/065 20130101; G03G 15/5062
20130101 |
Class at
Publication: |
399/27 ; 399/50;
399/55 |
International
Class: |
G03G 15/08 20060101
G03G015/08; G03G 15/02 20060101 G03G015/02 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 10, 2007 |
JP |
2007-102984 |
Claims
1. An image forming apparatus comprising: an image forming section
for forming an image by an electrophotographic process, the image
forming section including a photoconductor, a charging unit, a
developing unit, a toner supply unit which supplies a toner to said
developing unit, and a developing bias power supply section which
supplies a developing bias voltage to said developing unit; and a
controller that activates said image forming section to form the
image, determines a target value of a charging potential of said
photoconductor and/or a developing bias voltage, for forming the
image, and controls the charging unit and/or the developing bias
power supply section in accordance with a determined result,
wherein when an absolute value of the determined target value of
the charging potential or an absolute value of the determined
developing bias voltage is larger than a prescribed value, said
controller controls replacement of a prescribed amount of a toner
in the developing unit.
2. The image forming apparatus according to claim 1, further
comprising: a density measuring section for measuring a density of
the formed image, wherein when a prescribed opportunity comes, said
controller activates the image forming section to form an image of
a pattern having the prescribed amount of a toner, activates the
density measuring section to measure a density of said image,
calculates a target value of the charging potential and a
developing bias voltage, based on the measured result, controls
subsequent image formation based on the calculated result, and
determines whether or not the toner is replaced before a next image
is formed.
3. The image forming apparatus according to claim 1, wherein said
controller activates the image forming section to form an image of
a pattern using said prescribed amount of a toner, then activates
the toner supply unit to replenish the developing unit with a new
toner so as to make the replacement of the toner.
4. The image forming apparatus according to claim 3, wherein said
pattern using the prescribed amount of the toner has a width almost
equal to a maximum width which can be developed, and is a
substantially uniform halftone or dot-shaped pattern.
5. The image forming apparatus according to claim 3, wherein said
pattern using the prescribed amount of the toner is in a prescribed
size.
6. The image forming apparatus according to claim 3, further
comprising: a transfer section that transfers the image formed by
the image forming section to a printing sheet; and a transferring
power supply section that is capable of applying a transfer voltage
to the transfer section, wherein said controller controls the
transferring power supply section so that the transfer section
floats potentially or a voltage of a polarity which is the same as
a charging polarity of the toner is applied to the transfer
section, while said pattern using the prescribed amount of the
toner passes through the transfer section.
7. The image forming apparatus according to claim 6, wherein said
transfer section has a transfer member coming in contact with a
surface of the photoconductor, and said controller controls the
transferring power supply section so that the voltage of the same
polarity as the charging polarity of the toner and the voltage of
an absolute value larger than that of the charging potential of the
photoconductor is applied to said transfer member, after said
pattern using the prescribed amount of the toner passes through the
transfer section.
8. The image forming apparatus according to claim 7, wherein said
photoconductor is formed in an endless shape to rotate when an
image is formed, and said controller controls the transferring
power supply section so that said voltage is applied to said
transfer member, while the photoconductor rotates two or more times
after said pattern using the prescribed amount of the toner passes
through the transfer section.
9. The image forming apparatus according to claim 1, further
comprising: a toner coverage ratio recognizing section that
recognizes a toner coverage ratio of an image before the image is
formed, wherein said controller controls the replacement of the
toner only when the recognized toner coverage ratio is under a
prescribed value, and the toner is not replaced when a toner
coverage ratio of an image to be formed is recognized and the
recognized toner coverage ratio is the same as the prescribed value
or more, even if the absolute value of the target value of the
charging potential or the absolute value of the developing bias
voltage according to said target value is a value in which a
process of replacing the toner is carried out.
10. The image forming apparatus according to claim 1, wherein said
controller controls the replacement of the toner such that the
toner is consumed while the developing unit is not replenished with
a new toner at first, and then the developing unit is replenished
with the new toner, in a process of replacing the toner.
11. The image forming apparatus according to claim 10, wherein said
controller controls the replacement of the toner such that the
consumption of the toner is discontinued when a toner density in
the developing unit decreases to a prescribed lower limit while the
process of replacing the toner is carried out.
12. The image forming apparatus according to claim 11, wherein said
controller controls the replacement of the toner such that the
developing unit is replenished with a new toner after the
consumption of the toner is discontinued, and then the toner is
consumed again.
13. The image forming apparatus according to claim 1, further
comprising: a transfer section that transfers the image formed by
the image forming section to a printing sheet; and a sheet supply
section that supplies a printing sheet to the transfer section,
wherein said controller further controls said sheet supply section
so that said printing sheet is not supplied to the transfer section
while a process of replacing the toner is carried out.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is related to Japanese application No.
2007-102984 filed on Apr. 10, 2007 whose priority is claimed under
35 USC .sctn. 119, the disclosure of which is incorporated by
reference in its entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an image forming apparatus
capable of controlling a charging potential of a photoconductor and
a developing bias voltage in an electrophotographic process, and
having a function of adjusting a density of a formed image.
[0004] 2. Description of the Related Art
[0005] When an image with a low toner coverage ratio, namely, an
image having few parts where toner is adhered out of an entire area
of a print image, is printed continuously, it is known that a
granular "fog" (phenomenon that the toner is adhered to a white
background part where the toner is not supposed to be adhered) is
generated eventually. Although a cause of this type of fog is not
clarified, it is empirically known that after printing at a low
toner coverage ratio is continued to some extent, the fog is
generated. From this fact, it is estimated that when the same toner
is retained in a developing unit, a kind of deterioration occurs to
the toner, thus causing the fog.
[0006] Generation of the fog is not preferable in terms of image
quality. Therefore, a technique of preventing the fog is proposed,
in such a way that the fog on the photoconductor caused by the
deterioration of the toner is detected by an optical sensor, and
when the fog is generated, a toner image is formed on both ends in
a direction of a photoconductor rotation axis outside of an image
area, and the toner is forcibly discharged to replace the toner
(for example see Japanese Unexamined Patent Publication No.
2006-243115).
[0007] However, since the fog is a phenomenon in which the toner is
extremely thinly applied to a non-image area, it is difficult to
stably detect the fog with accuracy. In addition, it may be
preferable to predict the generation of the fog and then cope with
the fog, rather than coping with the fog after actually it is
generated.
[0008] As a result of earnest efforts to study on a condition of
allowing the fog to be generated after printing at a low toner
coverage ratio, inventors of the present invention find a point
that there is a correlation between the charging potential of the
photoconductor and the generation of the fog. Namely, it is found
that the larger an absolute value of the charging potential of the
photoconductor is, the more easily the fog is generated. Moreover,
it is found that when the toner is retained for a long period in
the developing unit, the density of the image hardly appears. As a
result, it is found that an image density adjustment process
control executed so as to stabilize the image density makes an
absolute value of the charging potential of the photoconductor
large, thus leading to a circumstance where the fog is easily
generated.
[0009] In addition, it is confirmed that there is a correlation
between a use period of the photoconductor and the generation of
the fog. Namely, it is confirmed that when the photoconductor is
new, the fog is hardly generated, and as the use period is elapsed,
the fog is easily generated.
[0010] Neither deterioration of the image density nor the fog is
preferable, in terms of the image quality. The deterioration of the
image density and the fog must be suppressed, so as not to be
recognized by a user. However, in a case where the charging
potential is controlled to stabilize the image density when the
printing at a low toner coverage ratio is continued as described
above, the fog is easily generated. Accordingly, there is desired a
technique capable of accurately predicting or determining a
condition where the fog is easily generated, and a technique
capable of appropriately coping with such a condition.
SUMMARY OF THE INVENTION
[0011] The present invention is provided in view of the
above-described circumstances, and the present invention is
directed to providing a technique capable of accurately predicting
the fog generated after printing at a low toner coverage ratio is
continued. In addition, from a viewpoint different from the above,
the present invention is directed to providing a technique capable
of determining a condition where the fog is easily generated
without requiring extra cost and time.
[0012] The present invention provides an image forming apparatus
including: an image forming section for forming an image by an
electrophotographic process, the image forming section including a
photoconductor, a charging unit, a developing unit, a toner supply
unit which supplies a toner to said developing unit, and a
developing bias power supply section which supplies a developing
bias voltage to said developing unit; and a controller that
activates said image forming section to form the image, determines
a target value of a charging potential of said photoconductor
and/or a developing bias voltage, for forming the image, and
controls the charging unit and/or the developing bias power supply
section in accordance with a determined result, wherein when an
absolute value of the determined target value of the charging
potential or an absolute value of the determined developing bias
voltage is larger than a prescribed value, said controller controls
replacement of a prescribed amount of a toner in the developing
unit.
[0013] According to an image forming apparatus of the present
invention, a controller controls so that a prescribed amount of
toner in a developing unit is replaced, when a determined target
value of a charging potential becomes larger than a prescribed
value as an absolute value. Therefore, it is possible to accurately
determine a condition where the fog is easily generated in terms of
an image forming condition and a generation of the fog can be
prevented by replacing at least a part of the toner in the
developing unit. In addition, it is possible to determine the
condition where the fog is easily generated without requiring extra
costs and time.
[0014] Alternately, the controller controls, so that the toner of a
prescribed amount in the developing unit is replaced, when an
absolute value of the decided developing bias voltage is larger
than a prescribed value. Therefore, it is possible to accurately
determine the condition where the fog is easily generated in terms
of an image forming condition, and the generation of the fog can be
prevented by replacing at least a part of the toner in the
developing unit. Further, it is possible to determine the condition
where the fog is easily generated without requiring extra cost and
time.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a graph illustrating a condition where a granular
fog is easily generated after printing at a low toner coverage
ratio according to the present invention;
[0016] FIG. 2 is an explanatory view illustrating a mechanical
structure of an electrophotographic printer according to one aspect
of an image forming apparatus of the present invention;
[0017] FIG. 3 is a sectional view illustrating structures of a
development section and a toner container of the
electrophotographic printer shown in FIG. 2;
[0018] FIG. 4 is a block diagram illustrating a structure of a
functional block regarding a control of an electrophotographic
process according to this embodiment;
[0019] FIGS. 5A to 5D are explanatory views schematically
illustrating an example of a charging potential, a potential of an
electrostatic latent image of the electrophotographic process
having a plurality of image densities, a developing potential and a
transfer voltage according to this embodiment;
[0020] FIGS. 6A to 6D are explanatory views illustrating an example
of an updated charging potential, an updated potential of an
electrostatic latent image of the electrophotographic process
having a plurality of image densities, an updated developing
potential and an updated developing potential according to this
embodiment;
[0021] FIG. 7 is a first flowchart illustrating an execution
procedure of a toner replacing process according to the present
invention;
[0022] FIG. 8 is a second flowchart illustrating the execution
procedure of the toner replacing process according to the present
invention; and
[0023] FIG. 9 is a graph of a test result showing effectiveness of
a control method according to this embodiment.
DETAILED DESCRIPTION OF THE INVENTION
[0024] In the present invention, the system, type, and structure of
the photoconductor, the charging unit, the developing unit, the
toner supply unit, the developing bias power supply section, which
configure the image forming section, are not particularly limited,
provided that they can be applied to the image forming apparatus of
an electrophotographic system. The controller may be realized, by
executing a control program showing a procedure of the processing
by, for example, a microcomputer or a CPU. However, the controller
is not limited thereto, and, for example may be realized only by a
circuit as hardware.
[0025] Preferred embodiments of the present invention will be
explained hereunder.
[0026] The image forming apparatus may further include: a density
measuring section for measuring a density of the formed image,
wherein when a prescribed opportunity comes, said controller may
activate the image forming section to form an image of a pattern
having the prescribed amount of a toner, may activate the density
measuring section to measure a density of said image, may calculate
a target value of the charging potential and a developing bias
voltage, based on the measured result, may control subsequent image
formation based on the calculated result, and may determine whether
or not the toner is replaced before a next image is formed With
this structure, after the charging potential and/or the developing
bias voltage are updated for stabilizing the image density, it is
determined whether or not executing the process (toner replacing
process) for replacing the toner before forming the next image.
Therefore, the timing for updating the image forming condition and
the timing for determining necessity for the toner replacing
process are synchronized with each other. Accordingly, the image
forming condition is not carelessly updated to allow the fog to be
generated, and the toner replacing process is not uselessly
executed.
[0027] The controller may activate the image forming section to
form an image of a pattern using said prescribed amount of a toner,
then may activate the toner supply unit to replenish the developing
unit with a new toner so as to make the replacement of the toner.
Thus, the replacement of the toner can be realized, without adding
a dedicated mechanism.
[0028] Also, the pattern using the prescribed amount of the toner
may have a width almost equal to a maximum width which can be
developed, and may be a substantially uniform halftone or
dot-shaped pattern. Thus, the toner can be uniformly consumed
approximately over an entire area of the developing unit. In
addition, by adjusting an average gradation value of a pattern, a
speed for consuming the toner can be set to a proper speed.
[0029] Still further, the pattern using the prescribed amount of
the toner may be in a prescribed size. Thus, a prescribed amount of
toner can be consumed by a single toner replacing process.
[0030] The image forming apparatus may further include a transfer
section that transfers the image formed by the image forming
section to a printing sheet; and a transferring power supply
section that is capable of applying a transfer voltage to the
transfer section, wherein said controller may control the
transferring power supply section so that the transfer section
floats potentially or a voltage of a polarity which is the same as
a charging polarity of the toner is applied to the transfer
section, while said pattern using the prescribed amount of the
toner passes through the transfer section.
[0031] Further, the transfer section may have a transfer member
coming in contact with a surface of the photoconductor, and said
controller may control the transferring power supply section so
that the voltage of the same polarity as the charging polarity of
the toner and the voltage of an absolute value larger than that of
the charging potential of the photoconductor may be applied to said
transfer member, after said pattern using the prescribed amount of
the toner passes through the transfer section. Thus, by applying
the voltage, the transfer member can be electrostatically
cleaned.
[0032] Still further, the photoconductor may be formed in an
endless shape to rotate when an image is formed, and said
controller may control the transferring power supply section so
that said voltage is applied to said transfer member, while the
photoconductor rotates two or more times after said pattern using
the prescribed amount of the toner passes through the transfer
section. Thus, the transfer member can be surely cleaned.
[0033] The image forming apparatus may further include: a toner
coverage ratio recognizing section that recognizes a toner coverage
ratio of an image before the image is formed, wherein said
controller may control the replacement of the toner only when the
recognized toner coverage ratio is under a prescribed value, and
the toner is not replaced when a toner coverage ratio of an image
to be formed is recognized and the recognized toner coverage ratio
is the same as the prescribed value or more, even if the absolute
value of the target value of the charging potential or the absolute
value of the developing bias voltage according to said target value
is a value in which a process of replacing the toner is carried
out. When it is known that the image with the toner coverage ratio
set at a prescribed value or more is printed next, the toner of the
developing unit is replaced by developing this image. In this case,
consumption of the toner can be suppressed, without daringly
executing the toner replacing process.
[0034] Also, the controller may control the replacement of the
toner such that the toner is consumed while the developing unit is
not replenished with a new toner at first, and then the developing
unit is replenished with the new toner, in a process of replacing
the toner. Thus, the toner in the developing unit can be
efficiently replaced.
[0035] Further, the controller may control the replacement of the
toner such that the consumption of the toner is discontinued when a
toner density in the developing unit decreases to a prescribed
lower limit while the process of replacing the toner is carried
out. Thus, it is possible to prevent the generation of a secondary
adverse effect that is generated when toner concentration is
excessively lowered, such as a drop of a carrier or damage of a
blade for cleaning the photoconductor.
[0036] Further, the controller may control the replacement of the
toner such that the developing unit is replenished with a new toner
after the consumption of the toner is discontinued, and then the
toner is consumed again.
[0037] In addition, the image forming apparatus may further
include: a transfer section that transfers the image formed by the
image forming section to a printing sheet; and a sheet supply
section that supplies a printing sheet to the transfer section,
wherein said controller may further control said sheet supply
section so that said printing sheet is not supplied to the transfer
section while a process of replacing the toner is carried out.
Thus, wasteful consumption of the sheet can be prevented.
[0038] A plurality of various preferable embodiments shown here can
be combined.
[0039] The present invention will be described in detail by using
the drawings. Note that explanation given hereunder is shown for
examples and should not be interpreted as restricting the present
invention.
Generation Condition of a Fog After Printing at a Low Toner
Coverage Ratio
[0040] First, explanation is given for a result of a test for
confirming correlativity between the fog that is generated after
printing at a low toner coverage ratio, and a charging potential.
Printing was performed with various grid voltages and a generation
circumstance of the fog was observed, so as to reproduce the
generation of a granular fog that is generated after printing at a
low toner coverage ratio. A size of an image is A4 size, and the
toner coverage ratio is 4.0%. A result is shown in FIG. 1. In FIG.
1, a horizontal axis indicates the number of print sheets, and a
vertical axis indicates the grid voltage. The grid voltage is
almost equal to a charging potential of a photoconductor drum. In
FIG. 1, an area surrounded by a gray circle shows an area in which
the generation of the fog is observed. When the number of print
sheets reaches almost 2,500 sheets, the fog is generated in the
area with the grid voltage set at Vg=-800 to -900V. Here, a
standard grid voltage is approximately -600V when a photoconductor
drum 202 and a developer are new. Also, a controllable range of the
grid voltage is 500V to 900V. Note that an evaluation of the fog is
performed, by sampling the toner adhered to a non-image part in
printing with an adhesive tape at a time when printing was
performed so that toner adhesion on the photoconductor was adjusted
to be 0.4 mg/cm.sup.2, and its image density (ID) was measured with
a color measurement color-difference meter (product name: by
X-Rite, X-Rite INC.). When the ID is 0.2 or less, this image
density is determined to be a defect.
[0041] As is shown in FIG. 1, after the printing at a low toner
coverage ratio is continued for a certain period, the fog to be
measured is generated. However, when the grid voltage is set in a
range from -500 to -800V, the fog to be measured is not generated
even in a case where the number of print sheets reaches near 2,500
sheets. It is found that when the absolute value of the grid
voltage is high, the fog is easily generated.
Overall Structure of the Image Forming Apparatus
[0042] Before the explanation is moved to a technique of
suppressing the fog, the explanation will be given for the
structure of the image forming apparatus, which is a base of this
technique. Namely, the structure of an image forming section of the
present invention will be explained.
[0043] FIG. 2 is a mechanical structure of an electrophotographic
printer, being one aspect of the image forming apparatus of the
present invention. In FIG. 2, an image forming apparatus 11 forms
an image of image data read by an image reading apparatus (not
shown) and print data inputted from external equipment (for
example, an image processing apparatus such as a personal computer)
via a communication line, then transfers and outputs the formed
image on a sheet for print (print sheet).
[0044] Each unit for electrophotographic process is disposed in the
image forming apparatus 11, with a photoconductor drum 202 as a
center, and the image is formed by an operation of them. The
photoconductor drum 202 is configured so that a photoconductive
layer is formed on a peripheral surface of a conductive base
material (such as aluminum). The base material is electrically
grounded to earth. A charger 203, a developing unit 200, a transfer
roller 207, cleaning unit 208, and an optical scanning unit 204,
etc, are disposed in this order, around the photoconductor drum
202. The photoconductor drum 202 is driven by a process drive motor
as will be described later (see FIG. 4), and is rotated at a
constant speed.
[0045] A surface of the photoconductor drum 202 is uniformly
charged by the charger 203. A scorotron-type charger 203 of the
present embodiment is a scorotron-type charger having a corona
discharge section and a control grid. The surface of the
photoconductor drum 202 is charged to a potential substantially
equal to the grid voltage. Note that other system such as a
charging roller may be used for the charger 203. The optical
scanning unit 204 functions to scan the surface of the uniformly
charged photoconductor drum 202 with optical beams to form an
electrostatic latent image on the surface. The developing unit 200
contains a developer inside thereof to develop the electrostatic
latent image written by the optical scanning unit 204 with toner.
Note that the developer is configured by toner and carrier, and by
being stirred in the developing unit 200, the toner is charged to a
positive polarity by friction with the carrier. A toner container
171 for containing the toner supplied to the developing unit 200 is
fitted to the developing unit 200.
[0046] The transfer roller 207 is a roller for transferring the
image developed on the photoconductor drum 202 to a print sheet
thereby to form a visible image on the sheet. The transfer roller
207 is formed of a metallic shaft member and a conductive elastic
material wound around its peripheral surface (such as EPDM and
urethane foam). The transfer roller 207 is driven by the process
drive motor, and a voltage from a transfer power supply as will be
describe later is applied to the shaft member of the transfer
roller 207. A transfer belt 206 extending to a lower stream side in
a feeding direction is mounted on the transfer roller 207. The
transfer belt 206 is configured by resin or rubber having
conductivity so that a volume resistance rate has a prescribed
value (for example, in a range of 1.times.10.sup.9 to
1.times.10.sup.13.OMEGA..cm).
[0047] The cleaning unit 208 removes the developer remained on the
photoconductor drum 202.
[0048] A sheet feeding tray 201 incorporated in the image forming
apparatus 11 is disposed in a lower part of the image forming
apparatus 11. The sheet feeding tray 201 is a tray for housing
print sheets. The print sheets contained in the sheet feeding tray
201 are separated one by one by a pickup roller 209, and the
separated sheet is then fed to a registration roller 210, and is
sequentially fed between the transfer roller 207 and the
photoconductor drum 202 in synchronization with the timing of the
image formed on the photoconductor drum 202 with the registration
roller 210. The voltage for transfer (transfer voltage) is applied
to the transfer roller 207. The toner developed and adhered to the
photoconductor drum 202 is transferred to the sheet by the transfer
voltage.
[0049] A fuser unit 205 is disposed in the image forming apparatus
11. The fuser unit 205 is a nip part where a heat roller 211 and a
pressure roller 212 are brought into contact with each other, so
that the toner transferred to the sheet is melted by heat and is
fused to the sheet by pressure.
[0050] The sheet passing through the fuser unit 205 is further fed
and ejected to a sheet exit tray 213.
[0051] Note that in FIG. 2, a monochromatic image forming apparatus
is exemplified. However, the present invention is not limited
thereto and can be applied to a full color image forming
apparatus.
Structure of a Developing Unit
[0052] In this embodiment, details of the developing unit 200 and
the toner container 171 of the aforementioned image forming
apparatus will be explained. FIG. 3 is a sectional view showing the
details of the developing unit 200 and the toner container 171 of
the image forming apparatus 11 shown in FIG. 2. As shown in FIG. 3,
a developing roller 187 of the developing unit 200 is disposed so
as to be opposed to the surface of the photoconductor drum 202. The
developing roller 187 supplies the toner to the surface of the
photoconductor drum 202 to adhere the toner to the electrostatic
latent image for developing the adhered toner. The developing
roller is driven by the aforementioned process drive motor. In
addition, the surface of the developing roller is configured by a
non-magnetic conductive member (such as an aluminum material), and
the voltage is applied to this conductive member from a transfer
power supply as will be described later (see FIG. 4). In the
developing unit 200, a toner concentration sensor 186 detects toner
concentration, so as to constantly supply the toner of prescribed
concentration to a part around the developing roller. The
controller (not shown) obtains an output of the toner concentration
sensor 186 and controls supply of the toner. The toner is supplied
from the toner container 171. The toner container 171 is configured
by a toner hopper 178 for stirring the toner and a toner bottle
loading part 172 for loading a cylindrical toner bottle 174. The
toner bottle 174 is loaded by a user. The toner is contained inside
of the toner bottle 174. The toner in the toner bottle 174 is fed
to a toner supply port 173 by a toner feeding mechanism not shown.
The toner feeding mechanism is driven by a toner feeding motor as
will be described later (see FIG. 4), to feed the toner.
[0053] The toner fed from the toner supply port 173 is guided into
the toner hopper 178. A stirring roller 175 driven by the toner
feeding motor as will be described later (see FIG. 4) to rotate in
a direction indicated by an arrow J1 is disposed in the toner
hopper 178. The stirring roller 175 stirs the toner, so that
fluidity is kept uniform, and feeds the toner to a toner storage
part 179 near the toner supplying roller 176. In addition, a toner
feeding sensor 177 is disposed in the toner storage part 171. The
toner feeding sensor detects the toner in the toner hopper 178
which is lower than a prescribed amount to generate a signal for
replenishing the toner from the toner bottle. The toner feeding
sensor 177 is a light reflection sensible sensor, being a sensor
for detecting an existence/non-existence of the toner in the toner
hopper 178, by irradiating an object to be detected with light and
determining a state of the object with a reflection degree of
light. The controller (not shown) controls an operation of the
toner feeding mechanism in accordance with the output of the toner
feeding sensor 177. Thus, the toner amount in the toner hopper 178
is maintained in a prescribed range.
[0054] The toner supplying roller 176 is a roller for supplying a
prescribed amount of toner to the developing unit 200. The toner
supplying roller 176 is formed by having a porous resilient member
such as ester-based polyurethane, being a so-called porous
resilient member such as a sponge, wound on a solid shaft made of
stainless. A slit-shaped toner drop opening part 183 is formed in a
lower part of the toner supplying roller 176, so as to communicate
with the developing unit 200. The toner supplying roller 176 is
disposed so as to cover an entire surface of the toner drop opening
part 183 with its porous resilient member. In addition, the toner
supplying roller 176 is driven by the toner supply motor and is
rotated in a direction indicated by an arrow J2.
[0055] When the toner supplying roller 176 is rotated, the toner of
the toner storage part 179 enters a hole part of the surface of the
porous resilient member. When this toner reaches the toner drop
opening part 183, the surface of the toner supplying roller 176 is
brought into contact with an edge of the toner drop opening part
183 and is deformed. With this deformation, the toner is separated
from the hole part, and drops to an inside of the developing unit
200 from the toner drop opening part 183 by its own weight.
[0056] When the toner supplying roller 176 stops, the entire
surface of the toner drop opening part 183 is covered with the
porous resilient member of the toner supplying roller 176.
Accordingly, in a state where the toner supplying roller 176 is
stopped, the toner in the toner hopper 178 is prevented from moving
to the developing unit 200.
[0057] The toner that drops into the developing unit 200 from the
toner drop opening part 183 is carried by a carrying screw 184 in a
development bath, and is stirred with the carrier by a stirring
screw 185, and is fed to a surface part of the developing roller
187 by an action of the stirring screw 185.
[0058] A toner concentration sensor 186 is provided at a bottom
part of the developing unit 200. The toner concentration sensor 186
detects a concentration of the toner fed to the surface part of the
developing roller 187. Here, the concentration of the toner refers
to a ratio of a weight of the toner over the weight of the
developer which is formed by combining the carrier and the toner.
When the electrostatic latent image on the photoconductor drum 202
is developed, the toner is consumed. When reduction of the toner in
the developing unit is recognized by the signal from the toner
concentration sensor 186, the controller (not shown) rotates the
toner supply motor. When the toner supply motor is rotated, the
stirring roller 175 and the toner supplying roller 176 are rotated
and the toner is supplied into the developing unit 200. Moreover,
the controller stops the toner supply motor when the toner
concentration reaches a prescribed value. Thus, the toner
concentration in the developing unit 200 is controlled in a
prescribed range.
Control of Electrophotographic Process
[0059] Next, explanation will be given for a functional structure
for controlling an image forming condition of an
electrophotographic process with the image forming apparatus 11 in
FIG. 2. Namely, the controller of the present invention will be
explained.
[0060] FIG. 4 is a block diagram illustrating the structure of a
functional block regarding a control of the electrophotographic
process according to this embodiment. In FIG. 4, an image forming
instruction section 80 is a block for sending an instruction of
image formation to a controller 81. When the image forming
apparatus 11 has a copy function, the image forming instruction
section 80 may send a signal showing a message that a copy start
key provided on an operation panel not shown of the image forming
apparatus 11 is pressed. Hardware of the controller 81 may be, for
example, a microcomputer. With the execution of a control program
by the microcomputer, a function of the controller 81 is realized.
The controller 81 recognizes a state where the copy start key is
pressed, as a start request of a copy job. The hardware of the
image forming instruction section 80 may be a key and a circuit of
the operation panel. In addition, when the image forming apparatus
11 has a function of a printer, the image forming instruction
section 80 may be a communication circuit for receiving a command
and print data from a host via a communication line. The controller
81 analyzes a content of the received command to recognize the
start request of the print job.
[0061] The controller 81 receives the start request of a job from
the image forming instruction section 80, being the instruction of
the image formation, and controls each block regarding the
electrophotographic process. The function of each block is as
follows.
[0062] In a case of a copy job, an image data creating section 82
is a block that processes image data of a document read by a
scanner, and creates the image data to be printed. In a case of a
printer, the image data creating section 82 is also a block that
develops the print data received from the host to create the image
data to be printed. Its hardware is configured by storage elements
such as an LSI and RAM, ROM, and nonvolatile memory.
[0063] An image data output section 83 processes the image data
created by the image data creating section 82 to create an output
signal to an optical scanning unit 204. Note that preferably, the
image data creating section 82 or the image data output section 83
have a function of providing a toner coverage ratio of each page to
form an image, prior to printing This function can be realized by a
circuit or a program for counting the number of print pixels of the
image data in a page unit.
[0064] The optical scanning unit 204 includes a laser light
emitting element 85 and a scan control circuit 84 for
PWM-modulating a light emitted by the laser light emitting element
85. The scan control circuit 84 PWM-controls on/off of the light
emission of the laser light emitting element 85 in accordance with
a signal inputted from the image data output section 83. The laser
light emitting element 85 emits laser beams which is PWM-modulated
by the scan control circuit 84 toward a peripheral surface of the
photoconductor drum 202. The laser beams are deflected by a polygon
mirror (not shown). The deflected laser beams scan the peripheral
surface of the photoconductor drum 202 along a direction of its
rotating shaft. The photoconductor drum 202 rotates along with a
rotation of a photoconductor drive motor 56. The peripheral surface
of the photoconductor drum 202 is selectively exposed to light in
cooperation of the scan of the modulated laser beams and the
rotation of the photoconductor drum 202, and an electrostatic
latent image is thereby formed. A process drive motor control
circuit 29 controls rotation, stop, and a rotation speed of a
process drive motor 31.
[0065] The controller 81 obtains a signal from the toner feeding
sensor 177 and outputs a control signal to a toner feeding motor
control circuit 21. The toner feeding motor control circuit 21
receives the control signal and controls the rotation and the stop
of a toner feeding motor 23. Also, a controller 81 obtains the
signal from a toner concentration sensor 186 and outputs a control
signal to a toner supply motor control circuit 25. The toner supply
motor control circuit 25 receives the control signal and controls
the rotation and the stop of a toner supply motor 27.
[0066] Further, the controller 81 controls, so as to turning on/off
a charging power supply 61, a developing bias power supply 62, a
transfer power supply 63 at a prescribed timing. The charging power
supply 61 applies a discharge voltage to a corona discharge section
of a scorotron-type charger 203, and also applies a grid voltage to
a control grid. The grid voltage, an output voltage of the
developing bias power supply 62 (developing bias voltage), and an
output voltage of the transfer power supply 63 (transfer voltage)
are variable and controlled by the controller 81. Note that a
ground potential is set as a reference of the grid voltage, the
developing bias voltage, and the transfer voltage. Next, a voltage
control of them will be explained.
Potential Control of a Process
[0067] In this embodiment, explanation will be given for a
potential control of the electrophotographic process by the
controller and particularly the control of a charging potential and
the developing bias voltage. FIGS. 5A to 5D are explanatory views
schematically illustrating an example of a charging potential, a
potential of an electrostatic latent image of the
electrophotographic process having a plurality of image densities,
a developing potential and a transfer voltage according to this
embodiment. FIG. 5A shows that a charging potential Vs of the
peripheral surface of the photoconductor drum 202 by the charger
203 is set at -600V, against 0V, namely, the ground potential. The
charging potential Vs has an almost equal value to the value of the
grid voltage Vg. Vs=Vg is assumed to be established for simplifying
explanation hereunder.
[0068] FIG. 5B illustrates an example of the potential to each
gradation, when each area of the peripheral surface of the charged
photoconductor drum 202 is exposed to light by the PWM-modulated
laser beams in accordance with a contrast of the image. Each area
of the photoconductor drum 202 shows the potential according to the
contrast of the corresponding image. This is the electrostatic
latent image. The potential of the peripheral surface of the
photoconductor drum 202 corresponding to each gradation is called a
latent image potential. In FIG. 5B, arrow in a horizontal direction
shows a corresponding relationship between the latent image
potential and contrast of brightness of the image. A brightest part
of the image (usually, a white background part) is not exposed to
light. Accordingly, the charging potential of -600V is maintained
as the latent image potential of the white background part.
Meanwhile, a dark part of the image is most strongly exposed to
light. Thus, the potential of the peripheral surface of the
photoconductor drum 202 is lowered toward the ground potential. In
an example of FIG. 5B, the latent image potential of the dark part
is -50V. A change of the latent image potential between a bright
part and a dark part, namely, gradation characteristics or y
characteristics shows soaring characteristics. Note that a
step-shaped graph shows that a resolution of the PWM modulation is
a finite discrete value. However, it can be said that the
resolution of the PWM modulation is substantially a continuous
value.
[0069] Developing bias voltage Vdv from the developing bias power
supply 62 is applied to the developing roller 187. Therefore, the
surface of the developing roller 187 shows the potential
(developing potential) equal to the developing bias voltage. The
developing potential is controlled to -500V closer to the dark part
against the latent image potential -600V of the white background
part. A potential difference from the latent image potential of the
white background part is provided for surely preventing an adhesion
of the toner to the white background part. The adhesion of the
toner means the "fog " in a broad sense. The toner is charged to a
negative polarity by a friction with the carrier. The toner of an
amount according to a difference between the developing potential
and the latent image potential is adhered to an area of the
electrostatic latent image having a positive latent image potential
as a reference of the developing bias voltage.
[0070] The transfer roller 207 and the transfer belt 206 have
conductivity. When the print sheet passes through the area
(transfer area) sandwiched between the photoconductor drum 202 and
the transfer belt 206, the controller 81 controls the transfer
power supply 63 so that the transfer voltage Vt of -2 kV is applied
to the transfer roller 207. The print sheet has an insulating
property. In the transfer area at this time, a capacitor is formed,
with the base material of the photoconductor drum 202 set as one of
the electrodes, and the transfer roller 207 and the transfer belt
206 set as the other electrode. The toner in the transfer area is
transferred to the print sheet from the surface of the
photoconductor drum 202 by an action of an electrical field
generated by the transfer voltage.
[0071] Each potential and a value of the voltage is an example in
the aforementioned explanation. It is a usual example that the
image forming apparatus in recent years has an image density
correcting function, being a function of a so-called process
control, for stabilizing the density of the image. In the image
density correction, the controller 81 forms and develops a test
pattern for measuring density. Then, by using a density measurement
section (not shown), the density of the developed test pattern is
measured on the surface of the photoconductor drum 202 or on the
transfer belt 206. Then, based on a measurement result, the
controller 81 determines values of the grid voltage, the developing
bias voltage, and the transfer voltage. The following image
formation is performed by using each determined voltage.
[0072] For example, when it is so determined that the density is
low as a result of measuring the density of the test pattern, the
controller 81 determines an appropriate charging potential in
accordance with the measurement result. Further, the developing
potential according to the updated charging potential is
determined. FIGS. 6A to 6D are explanatory views illustrating an
example of an updated charging potential, an updated potential of
an electrostatic latent image of the electrophotographic process
having a plurality of image densities, an updated developing
potential and an updated developing potential according to this
embodiment. In the example of FIG. 6, the updated charging
potential is -800V, and the updated developing potential is -700V.
Namely, the developing potential is updated so as to maintain the
potential difference (100V) between the charging potential and the
developing potential. Based on the updated charging potential and
the developing potential, the controller 81 controls the grid
voltage Vg and the developing bias voltage Vdv, in the following
image formation.
[0073] Generally, along with a use of a developer and the
photoconductor drum 202, the density of the image is lowered.
Accordingly, the function of the process control has a tendency of
setting an absolute value of the grid voltage to be large with a
lapse of time. Along with this tendency, the absolute value of the
developing bias voltage is also set to be large. When the developer
and/or the photoconductor drum 202 is replaced with a new one, the
absolute value of the grid voltage and developing bias voltage
become smaller. However, the aforementioned tendency is a general
tendency, and the absolute values of the grid voltage and the
developing bias voltage are not always updated in a direction where
the absolute values of the grid voltage and the developing bias
voltage are set large. The values of a new grid voltage and the
developing bias voltage depend on the measurement result of the
density.
Control Method for Suppressing Fog Generation
[0074] A suppressing method of the fog according to this invention
will be explained. Based on a test result of FIG. 1, inventors of
the present invention conceive of a control as follows. Namely, as
a result of continuing the printing at a low toner coverage ratio
and correcting the image density, the toner in a developing tank is
replaced, when a target absolute value of the grid voltage becomes
large up to a prescribed value. Alternately, the toner in the
developing tank is replaced when the absolute value of the
developing bias voltage becomes large up to the prescribed value
instead of the grid voltage. In correcting the image density, a
target value of the developing bias voltage is determined,
depending on the target value of the grid voltage. Therefore, it
can be so considered that both of the developing bias voltage and
the grid voltage have the same result.
[0075] The toner is preferably replaced when the density of the
test pattern is measured in correcting the density of the image.
Specifically, the toner is preferably replaced by forming the image
of a halftone or a halftone dot pattern as a toner discharging
pattern, and developing this image.
[0076] Note that while the formed toner discharging pattern passes
through the transfer area, preferably, electrical floating of the
transfer roller 207 is executed for reducing or preventing a stain
of the transfer belt 206, or the transfer voltage of the same
polarity as the charging polarity of the toner is applied to the
transfer roller 207.
[0077] Further, after the formed toner discharging pattern passes
through the transfer area, preferably the voltage, having the same
polarity as the charging polarity of the toner and a larger
absolute value than that of the charging potential of the
photoconductor is applied as a transfer voltage.
[0078] A detailed procedure of the toner replacing process will be
explained hereunder. FIG. 7 and FIG. 8 are flowcharts showing the
procedure of executing the toner replacing process by the
controller 81.
[0079] First, in FIG. 7, the control of the controller 81 executes
correction of image density after turning on the power, and then
updates the values of the grid voltage and the developing bias
voltage in the image formation (step S11). Then, the image
formation of a series of pages, namely the start instruction of a
job is awaited (step S13). As described above, a copy job and a
print job, etc, for example, are given as the types of the job.
When the start instruction of the job is received, first, the
controller 81 determines whether or not the timing for executing
the image density correction arrives (step S15). This is because
the image density correction is intermittently executed, for
example for each previously defined number of print pages or elapse
of a period. When a time opportunity for performing image density
correction does not arrive yet, the routine is advanced to step
S19. Meanwhile, when the timing for the image density correction
arrives, the controller 81 executes the image density correction,
and then updates the values of the grid voltage and the developing
bias voltage in the image formation (step S17).
[0080] Next, the controller 81 determines whether or not history
data DRh at an average toner coverage ratio of the image printed in
the past prescribed period is under a previously defined allowable
value L1 (step S19). Here, the history data CRh is the data stored
in the nonvolatile memory. When a determined result is No, the
routine is advanced to step S29, and print of each page is started.
The meaning of the determined result of No is that the past
printing is performed at the toner coverage ratio of the allowable
value or more, thus providing a circumstance where the fog is
hardly generated. Meanwhile, when the determined result is Yes, the
controller 81 acquires toner coverage ratio CRj of the job for
printing from now, from the image data creating section 82 or the
image data output section 83 (step S20). Then, whether or not the
toner coverage ratio CRj is a previously defined allowable value L2
or less is determined (step S21). The allowable value L2 is the
value previously defined according to a size of a page to be
printed, the number of pages, and the toner coverage ratio of each
page. When the determined result is No, the routine is advanced to
step S29, and the print of each page is started. The meaning of the
determined result of No is that the toner of a prescribed amount or
more is consumed in the next print. Note that when the data to be
printed at a processing time point of steps S20 and S21 is not
acquired yet, the routine is advanced in a direction of the
determined result of Yes. The same thing can be said for a case
that the image data creating section 82 and the image data output
section 83 have no capability of providing the toner coverage ratio
CRj.
[0081] When the determined result is Yes, the controller 81
acquires an output value Vg of the grid voltage determined based on
the image density correction, and determines whether or not its
absolute value is larger than a previously defined threshold value
Lg (step S23). When the determined result is No, the routine is
advanced to step S29, and the print of each page is started The
meaning of the determined result of No is that the absolute value
of the grid voltage Vg (namely, the charging potential Vs of the
photoconductor) is the allowable value or less, thus providing the
circumstance where the fog is hardly generated. When the determined
result is Yes, the controller 81 executes the toner replacing
process to replace the toner in the developing unit 200 (step S25).
The detailed procedure of the toner replacing process will be
described separately later.
[0082] After the toner replacing process is ended, the controller
81 executes the image density correction again. Then, new grid
voltage and developing bias voltage are obtained. After the toner
replacing process is executed, a deteriorated toner is discharged,
then the density of the image easily appears, and the absolute
values of the grid voltage and the developing bias voltage are
generally made smaller.
[0083] Thereafter, the controller 81 starts the print of each page
(step S29). When the print up to a final page of the job is ended
(step S31), the controller 81 updates the history data CRh at an
average toner coverage ratio (step S33). Namely, the toner coverage
ratio CRj of each page printed by executing the job is reflected on
the history data CRh. Here, the history data CRh is the past
average toner coverage ratio ranging over prescribed pages. Out of
these pages, the page printed this time, namely the page being a
target of the toner coverage ratio CRj, is added to the history
data and an old page in the history data is deleted from the object
of the history data, so as to cancel the portion of the added page.
Then, the average toner coverage ratio of the object page of the
updated history data is calculated and maintained as the value of
the updated CRh.
[0084] Subsequently, a detailed procedure of the toner replacing
process will be explained based on FIG. 8. In FIG. 8, first,
regarding the grid voltage Vg and the developing bias voltage Vdv,
the controller 81 maintains the voltage based on the image density
correction and outputs the voltage equal to the developing bias
voltage Vdv, as the transfer voltage Vt (step S41). The transfer
voltage Vt is set at a prescribed voltage (-2kV in FIG. 5 and FIG.
6) during forming the image at a timing when the print sheet passes
through the transfer area. Then, although set at 0V at other
timing, the transfer voltage Vt is set at the voltage equal to the
developing bias voltage Vdv during the toner replacing process.
[0085] Then, exposure of the optical scanning unit 204 is started
to form the toner discharging pattern on the photoconductor drum
202. The formed toner discharging pattern is then developed to
consume the toner in the developing unit 200 (step S43). An area
for forming the toner discharging pattern is previously defined.
This is because the toner amount consumed by the toner replacing
process is approximately determined by this area and the density of
the toner discharging pattern.
[0086] During developing the drum discharging pattern, the
controller 81 monitors the toner concentration and determines
whether or not the toner concentration is below a control target
value Ld of the toner concentration by a previously defined margin
a (step S45). Here, the target value Ld is the value for
controlling the supply of the toner by the controller 81, to
maintain the toner concentration in the developing unit 200. As
described above, the toner amount consumed by the toner replacing
process is approximately fixed, and the toner concentration after
the toner replacing process is also fixed. However, there is a
variation depending on a surrounding environment such as
temperature and humidity and the toner concentration at the time of
starting the toner replacing process. When the toner concentration
is excessively lowered, a trouble such as a drop of the carrier
from the developing unit occurs. Therefore, the controller 81
checks so that the toner concentration is not lowered beyond the
margin .alpha..
[0087] When the determined result of step S45 is Yes, consumption
of the toner is continued, and end of forming the toner discharging
pattern is awaited (step S61). When the determined result is No,
namely, when the toner concentration is excessively lowered, the
routine is advanced to step S47. Here, the controller 81 interrupts
the exposure of the toner discharging pattern (step S47),
calculates the area of the toner discharging pattern to be formed
when image formation is restarted later, the calculated area of the
toner discharging pattern is then temporarily maintained (step
S49). The area may be the remaining area of the toner discharging
pattern at the time point of interruption. However, the area is
preferably calculated and corrected in consideration of the toner
amount replenished thereafter. Then, the controller 81 replenishes
the toner in the developing unit 200 to increase the toner
concentration (step S51). Then, recovery of the toner concentration
up to the value smaller than the target value Ld by
.beta.(.beta.<.alpha.) is awaited (step S53). Here, .beta. may
be a previously defined value or may be calculated by the
controller 81 in step S49, according to the remaining area of the
toner discharging pattern.
[0088] When the toner concentration recovers up to Ld to .beta.,
the controller 81 restarts the exposure of the toner discharging
pattern (step S55). The toner discharging pattern formed thereafter
is the area calculated in the aforementioned step S49. After the
exposure of the toner discharging pattern is restarted, the routine
is advanced to step S45, and the end of the toner discharging
pattern is awaited while monitoring the toner concentration.
[0089] When the formation of the toner discharging pattern is
ended, the controller sets the transfer voltage Vt as a prescribed
cleaning voltage Vc (step S65) in a state of not exposing the
photoconductor drum 202 with the optical scanning unit 204, and
waits until the transfer belt 206 goes 2 rounds (step S67). The
cleaning voltage Vc is the voltage of the same polarity as the
charging polarity of the toner. As an example, Vc=+450V is
established. Thus, the toner adhered to the surface of the transfer
belt 206 is transferred to the photoconductor drum 202 side and the
transfer belt 206 is cleaned. Note that the toner transferred to
the photoconductor drum 202 is retrieved by the cleaning unit 208.
While the transfer belt 206 goes 1 round or more (2 rounds in the
embodiment of FIG. 8), the transfer belt 206 is cleaned.
[0090] In addition, the controller replenishes a new toner to the
developing unit 200 to increase the toner concentration (step S69).
Then, the recovery of the toner concentration up to a target value
Ld is awaited (step S71). Note that replenishment of the toner may
be performed in parallel to the cleaning of the transfer belt 206.
When the toner concentration is recovered, the transfer voltage is
turned off (step S73).
Test Result
[0091] FIG. 9 is a graph of a test result showing an effectiveness
of a control method according to this embodiment. In FIG. 9, a
horizontal axis indicates the number of print sheets, and a
vertical axis indicates the grid voltage Vg. The image at the toner
coverage ratio of 0.4% is printed and the grid voltage Vg that
varies with process control is plotted in this figure. A square
connected by a solid line shows a conventional control method. A
hollow square connected by a chain line shows the control method
according to this embodiment.
[0092] In the image forming apparatus used in a test, a process
speed is set at 350 mm/second, volume of the developer in the
developing unit is set at 900 g by mass, the target value Ld of the
toner concentration is set at 5.0%, a lower limit value of the
toner concentration is set at 4.5% (namely, a=0.5%), the amount of
the toner consumed by a single toner replacing process is set at 6
g by mass, and the allowable value L1 of the toner coverage ratio
is set at 0.5%. Note that the absolute value of the grid voltage
has a controllable upper limit of 850V, and the absolute value of
the developing bias voltage has a controllable upper limit of
700V.
[0093] As shown in FIG. 9, when the printing at a low toner
coverage ratio is continued by the conventional control method, the
toner is deteriorated and the image density is lowered. Therefore,
when the process control is executed, the absolute value of the
grid voltage becomes large. In FIG. 9, the absolute value of the
grid voltage reaches 850V, being an upper limit value in the
vicinity of 600 sheets, and this upper limit value is maintained
thereafter.
[0094] Meanwhile, in the control method according to this
embodiment, when the grid voltage is increased beyond a threshold
value Lg=825V, the toner replacing process is executed and the grid
voltage drops. In FIG. 9, the grid voltage exceeds the allowable
value over four times of A, B, C, D, and the toner replacing
process is executed. After the toner replacing process is executed,
the density of the image easily appears. Therefore, the absolute
value of the grid voltage becomes small. As a result, even in a
case where the number of print sheets reaches near 2,500 sheets,
being the number of print sheets where the fog is easily generated,
the grid voltage can be below the upper limit value.
[0095] In addition, in FIG. 9, as the toner replacing process is
repeated, the absolute value of the grid voltage is less frequently
made small. This is because the toner amount consumed in the toner
replacing process is small. Accordingly, by optimizing the amount
of the toner to be consumed, the effect could be further
maintained.
[0096] Various types of modified examples are possible in addition
to the above-described embodiments. These modified examples should
not be interpreted as not belonging to the scope of the claims of
the present invention. All modifications in the scope of the claim
and in the meaning equal to the scope of the claim should be
included in the present invention.
* * * * *